The present invention relates to a NAND memory architecture including resistive memory cells.
Resistive random-access memories (RRAMs) are a type of resistive memory that has generated significant interest recently as a potential candidate for ultra-high density non-volatile information storage. A typical RRAM device has an insulator layer provided between a pair of electrodes and exhibits electrical pulse-induced hysteretic resistance switching effects.
The resistance switching has been explained by the formation of conductive filaments inside the insulator due to Joule heating and electrochemical processes in binary oxides (e.g., NiO and TiO2) or by redox processes for ionic conductors including oxides, chalcogenides, and polymers. The resistance switching has also been explained by field-assisted diffusion of ions in TiO2 and amorphous silicon (a-Si) films.
In the case of a-Si structures, electric field-induced diffusion of metal ions into the silicon leads to the formation of conductive filaments that reduce the resistance of the a-Si structure. These filaments remain after a biasing (or program) voltage is removed, thereby giving the device its non-volatile characteristic, and the filaments can be removed by reversing the flow of the ions back toward the metal electrode under the motive force of a reverse polarity applied voltage.
Resistive devices based on an a-Si structure, particularly those that are formed on polysilicon, typically exhibit good endurance or life cycle. However, the endurance of the resistive device can be shortened if excessive bias voltage is applied during the repeated write and erase cycles, in part due to Joule heating and to movements of an unnecessarily large number of metal ions in the a-Si structure. Furthermore, in general, RRAM device yield is affected by the electroforming process, during which a major part of the conducting path is formed inside the insulating switching layer by applying a larger voltage (or current) signal to the device.